5.5.3: Plant responses: investigating tropisms

Question 1

What are meristems?

Answer 1

Groups of immature cells that are still capable of dividing.

Question 2

Where are apical meristems and what are they responsible for?

Answer 2

• At the tips or apices of roots and shoots.

- Responsible for the roots and shoots getting longer.

Question 3

Where are lateral bud meristems found and what are they responsible for?

Answer 3

• In the buds.

- can give rise to side shoots.

Question 4

Where are lateral meristems found and what are they responsible for?

Answer 4

• Form a cylinder near the outside of roots and shoots.

- Responsible for roots and shoots getting wider.

Question 5

Where are intercalated meristems located in some plants and what are they responsible for?

Answer 5

• Between nodes, where the leaves and buds branch off the stem.
• Growth between the nodes is responsible for the shoot getting longer.

Question 6

How could you investigate geotropic responses?

Answer 6

• Control plant constantly spun (very slowly) by a klinostat to ensure the effect of gravity is applied equally to all sides of the plant.
• Experimental plant, klinostat not turned on so gravity is only applied to one side.
• In experimental plant, root bends downwards, shoot bends upwards.
• Control plant, both root and shoot grow horizontally.

Question 7

A series of classical experiments confirmed that a chemical messenger from the shoot tip is responsible for phototropic responses. Who conducted these experiments?

Answer 7

• Darwin

- Boysen-Jensen

Question 8

What did Darwin conclude?

Answer 8

shoot tip is responsible for phototropic responses.

Question 9

What did Boysen-Jensen conclude?

Answer 9

-Water and/or solutes need to able to move backwards from the shot tip for phototropism to happen.
-Permeable gelatine block inserted behind root tip→shoot still shows positive tropism.
Impermeable mica block inserted→No phototropic response.

Question 10

Outline Boysen-Jensen’s experiment.

Answer 10

Permeable gelatine block inserted behind root tip:
-shoot still shows positive tropism.
Impermeable mica block inserted
-No phototropic response.

Question 11

What did Went’s experiment demonstrate?

Answer 11

-Demonstrated that a chemical messenger existed and could stimulate a phototropic effect artificially.

Question 12

Outline Went’s experiment.

Answer 12

• Shoot tip placed on an agar block.
• Auxin diffuses from shoot tip into agar.
• Agar block stimulates growth.
• Offset blocks containing auxin stimulate curve growth.
• Blocks containing no auxin have no effect.

Question 13

How could you confirm the role of Auxin as the chemical messenger?

Answer 13

• By using a series of blocks impregnated with different concentrations of auxin created by serial dilution.
• This gives shoot curvature in proportion to the amount of auxin.

Question 14

How do auxins cause shoot growth?

Answer 14

• Auxins produced at the apex of the shoot.

- Travels to the cells in the zone of elongation causing them to elongate, making the shoot grow.

Question 15

Describe shoot growth when light is equal on all sides.

Answer 15

Auxin promotes shoot growth evenly.

Question 16

Describe shoot growth when light only on one side.

Answer 16

• Auxins are transported to the shaded side, causing cells there to elongate more quickly.
• The shoot bends towards the light.

Question 17

What is the extent of cell elongation proportional to?

Answer 17

The concentration of auxins.

Question 18

How does auxin cause cell elongtion?

Answer 18

• Auxin increases the stretchiness of the cell wall by promoting active transport of H+ by an ATPase enzyme on the plasma membrane, into the cell wall.
• The resulting low pH provides optimum temperature for expansins to work.

Question 19

How do expansins work?

Answer 19

By breaking bonds within the cellulose (at the same time, the increased hydrogen ions also disrupt hydrogen bonds within cellulose.
-The walls become less rigid and can expand as the cell takes in water.

Question 20

Which two enzymes are involved in the redistribution of auxin due to light?

Answer 20

phototropin 1 and phototropin 2.

Question 21

What promotes the activity of phototropin 1 and 2?

Answer 21

• Blue light, this is the main component of white light that causes the phototropic response.
• Therefore, there is lots of phototropin 1 activity on the light side and, but progressively less activity towards the dark side.
• This gradient causes the redistribution of auxins through their effect on PIN proteins.

Question 22

What are PIN proteins?

Answer 22

• Transmembrane proteins that can be found dorsally, ventrally or laterally on the plasma membrane of cells.
• They control the efflux of auxin from each cell, essentially sending auxin in different directions in the shoot depending upon their location on the plasma membrane.

Question 23

What is the activity of PIN proteins controlled by?

Answer 23

A different molecule, called PINOID.

• Phototropins affect the activity of PINOID, which then affects PIN activity.
• This is just a theory, recent evidence suggests this may only work for pulse-induced phototropism, with another independent mechanism able to operate in continuous light.

Question 24

How is auxin involved in the geotropic responses of roots?

Answer 24

In a root lying flat, Went discovered that auxin accumulates on the lower side, where it inhibits cell elongation, the upper side continues to grow and the root bends downwards.

Question 25

The effect of auxin in roots is in contrast to that in the shoot, where auxin promotes cell elongation on the lower side making the shoot lying flat bend upwards. Why is this?

Answer 25

• Toot and shoot cells in the elongation zone exhibit different responses to the same concentrations of auxin.
• The concentrations that stimulate shoot growth, inhibit root growth.